Circuit Description
In order to control exhaust emissions of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx), a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide, it also reduces NOx, converting it into nitrogen. The catalytic converter also has the ability to store oxygen. The engine control module (ECM) has the capability to monitor this process using a rear heated oxygen sensor (HO2S2) located in the exhaust stream past the TWC. The HO2S2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. The ECM monitors the catalyst efficiency by first allowing the catalyst to heat up, waiting for a stabilization period while the engine is idling, and then adding and removing fuel while monitoring the reaction of the HO2S2. When the catalyst is functioning properly, the HO2S2 response to the extra fuel is slow compared to the front heated oxygen sensor (HO2S1). When the HO2S2 response is close to that of the HO2S1, the oxygen storage capability or efficiency of the catalyst is considered to be bad, and the malfunction indicator lamp (MIL) will illuminate.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0420 Catalyst System Low Efficiency
Conditions for Running the DTC
| • | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0300, P0336, P0337, P0341, P0342, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0502, P0506, P0507, P0562, P1133, P1167, P1171, and P1404 are not set. |
| • | Before the ECM performs the idle test, the vehicle must be driven under the following conditions: |
| - | The calculated air flow into the engine is more than 7 g/s for more than 10 seconds for a manual transmission. |
| - | The calculated air flow into the engine is more than 11 g/s for more than 11 seconds for an automatic transmission. |
| • | The transmission is in neutral or drive at idle. |
| • | The engine is in closed loop fuel control. |
| • | The EVAP purge concentration is learned. |
| • | The engine is operating for more than 9.5 minutes. |
| • | The calculated airflow into the engine at idle is between 2.25-7.5 g/s for an automatic transmission. |
| • | The calculated airflow into the engine at idle is between 2.25-6.5 g/s for a manual transmission. |
| • | The throttle position (TP) is less than 1.5 percent. |
| • | The engine coolant temperature (ECT) is between 70-105°C (158-158°F). |
| • | The intake air temperature (IAT) is between -7 and +70°C (+19 and +221°F). |
| • | The barometric pressure (BARO) is more than 72 kPa. |
| • | The calculated catalytic converter temperature is between 500-850°C (932-1,562°F). |
| • | The long term fuel trim is learned. |
| • | The engine idle time is less than 1 minute. |
| • | The vehicle speed is less than 3 km/h (2 mph). |
| • | DTC P0420 is ran one time per ignition cycle when the above conditions are met for more than 4.5 seconds. |
The catalyst test is disable if the ECM detects one of the following conditions:
| • | The engine speed changes more than 100 RPM. |
| • | The A/C compressor clutch changes state. |
| • | The cooling fans change state. |
| • | Insufficient air/fuel ratio activity. |
Conditions for Setting the DTC
The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
The catalyst test may abort due to a change in the engine load. Do not change the engine load, i.e. the A/C, the coolant fan, or the heater motor, while a catalyst test is in progress.
An intermittent problem may be caused by a poor connection, rubbed-through wire insulation, or a wire that is broken inside the insulation.
Any circuitry that is suspected of causing the intermittent complaint should be thoroughly inspected for the following conditions:
| • | Backed-out terminals |
| • | Improper mating |
| • | Broken locks |
| • | Improperly formed |
| • | Damaged terminals |
| • | Poor terminal-to-wire connection |
Step | Action | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Engine Controls? | Go to Step 2 | |||||||
2 | Observe the DTC information on the scan tool. Are there any other DTCs set? | Go to Step 3 | |||||||
3 | Inspect for the following conditions:
Did you find and correct the condition? | Go to Step 5 | Go to Step 4 | ||||||
4 |
Important: Before replacing the 3-way catalytic converter (TWC), correct any conditions that may have damaged the converter. Replace the three-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust. Did you complete the replacement? | Go to Step 5 | -- | ||||||
5 | Was the customer's concern that the malfunction indicator lamp (MIL) flashes? | Go to DTC P0300 | Go to Step 6 | ||||||
6 |
Did the DTC fail this ignition? | Go to Step 2 | Go to Step 7 | ||||||
7 | Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed? | System OK |
